The research data for the last decade have revealed that, besides their immune systems, almost all living organisms have an additional defense system against invading pathogenic microorganisms. They produce antimicrobial peptides in their bodies and use them as a defense means against pathogenic microorganisms.
Since the discovery of cecropin, a novel antimicrobial peptide, in silkworm larvae as a result of a study on the defense mechanism of insects against the invasion of microorganisms, the importance of peptides as physiologically active materials has been greatly recognized.
Thus far, as many as about 450 antimicrobial peptides have been found from amphibians, insects, mammals, plants, microorganisms and fishes. These antimicrobial peptides are known to be different in their sizes and amino acid sequences, but similar in their antimicrobial mechanism. Representative antimicrobial peptides can be exemplified by cecropin, magainin, bombinin, defensin, tachyplesin, and buforin. They are composed of 17-24 amino acids, showing antimicrobial activity against a broad spectrum of microorganisms, including Gram-negative bacteria, Gram-positive bacteria, protozoa and fungi. Some of them are effective against both cancer cells and viruses. For instance, magainin, consisting of 23 amino acids, was isolated from the skin of an amphibian and is reported not only to defend against pathogenic bacteria, but to kill human lung cancer cells (Zasloff, M. (1987) Proc. Natl. Acad. Sci., U.S.A. 84, 5449-5453).
Most of the antimicrobial peptides kill target cells rapidly and specifically, and have unusually broad activity spectra (Park, C. B., et al., (1997) FEBS Lett. 411, 173-178; Park, C. B., et al., (1996) Biochem. Biophys. Res. Comm. 218, 408-413). In addition to the microbicidal activity, antimicrobial peptides are reported to have other functions such as promotion of wound healing, stimulation of monocyte chemotaxis and inhibition of cytokine response.
Mucosal surfaces of living organisms are under constant attack from microorganisms. However, invasive infections are rare, remain localized and heal rapidly. Recent reports have established antimicrobial peptides as host-defense effector molecules which protect the mucosal epithelia from the invading microbes (Bevins, C. L. (1994) Ciba Found. Symp., 186, 250-260). Examples of these agents of mucosal immunity include andropin from the ductal epithelial cells of the reproductive tract of Drosophila, magainins from the mucosal skin surface and the gastrointestinal tract of Xenopus laevis, buforin I from the Bufo bufo gargarizans, tracheal antimicrobial peptide from bovine tracheal mucosa, and pleurocidin from Pleuronectes americanus (Cole, A. M., et al., (1997) J. Biol. Chem., 272, 12008-12013). While many antimicrobial peptides have been reported to be in the mucosal layer of amphibians, insects and mammals (Park, C. B. et al., (1996) supra; Samakovlis, C., et al., (1991) EMBO J. 10, 163-169; Diamond, G., et al., (1991) Proc. Natl. Acad. Sci., U.S.A., 88, 3952-3956), only a few have been found in the mucosal layer of aquatic organisms (Park, C. B., et al., (1997) supra; Cole, A. M., et al., (1997) supra; Chen, H., et al., (1998) FEBS Lett., 236, 462-466).